1. Field of the Invention
The present invention relates to electric double-layer capacitors which are low in capacity deterioration and long in service life.
2. Description of the Prior Art
FIG. 1 illustrates an electric double-layer capacitor device 1, where reference numeral 2 denotes a pair of polarizable electrodes 2 disposed in its center.
The polarizable electrodes 2 are made of solid carbonaceous compact primarily comprising an activated carbon, and is isolated by an electrically insulating separator 3.
Designated by numeral 4 are gaskets for both ends of the electric double-layer capacitor device 1, and by 5 are current collectors 5 thermally bonded to top and bottom of the gaskets 4.
In this arrangement, the polarizable electrodes 2 and separator 3 are accommodated inside the gaskets 4, in the state in which they are impregnated with an electrolyte such as, normally, a 50% by weight dilute sulfuric acid.
The above-described electric double-layer capacitor utilizes an electric double layer formed in the interface of the polarizable electrodes and the electrolyte Electrical characteristics and characteristic deterioration of the capacitor therefore depend to a great extent upon the affinity between the surface of the activated carbon, which is the primary material of the polarizable electrodes, and the electrolyte, or more specifically, upon the affinity between the surface functional groups present on the surface of the activated carbon and the electrolyte.
In more detail, on the surface of the activated carbon there exist some functional groups containing sulfur, hydrogen, chlorine, and nitrogen, as well as surface functional groups containing oxygen or functional groups containing oxygen, such as acidic functional groups, neutral functional groups, and basic functional groups.
The acidic functional groups are carboxyl group and phenylic hydroxyl group, the neutral functional groups are carbonyl group and quinone group, and the basic functional groups are unknown in detail of their structure.
The surface functional groups of the above-described activated carbon are shown in FIG. 2 by way of their models, where character I indicates carboxyl group, II does carboxyl group present as lactone, III does phenylic hydroxyl group, and IV does quinone group.
When an aqueous acidic electrolyte such as sulfuric acid is used as the electrolyte for the electric double-layer capacitor, any activated carbon which have on its surface more functional groups other than basic functional groups or functional groups containing oxygen would involve reactions between the electrolyte and the surface functional groups, causing the capacity for the weight of activated carbon to be lowered. In contrast, any activated carbon having a larger quantity of acidic functional groups (total quantity of all acidic groups, i.e. the sum of the quantities of carboxyl groups+carboxyl groups present as lactone+phenylic hydroxyl groups), if used as in the above case, will offer a higher level of capacity.
The above-described case is disclosed in Japanese Patent Laid-Open Publication No. 82514/1989.
The case disclosed therein is such that the more the total quantity of all acidic groups, the higher the capacity and besides the lower the capacity deterioration over long time periods.
However, even if the total quantity of acidic groups is large and the initial capacity is high, there have appeared some cases where capacity deterioration involved is considerably large.
In order to give larger capacities to electric double-layer capacitors, it is necessary to enhance the affinity between polarizable electrodes and electrolytes. (Without any high affinity, an activated carbon having a larger surface area, even if used as the primary material of the polarizable electrodes, would result in a poor utility efficiency of its surface area).
In the Japanese Patent Laid-Open Publication No. 82514/1989, there is disclosed a case where, as means for enhancing the affinity, oxidation treatment is performed and surface functional groups containing oxygen are brought into presence on the surface of an activated carbon. The publication describes that if an activated carbon has a large quantity of acidic functional groups (total quantity of all acidic groups, i.e. the sum of the quantities of carboxyl group I, carboxyl group present as lactone II, and phenylic hydroxyl group III, as shown in FIG. 2 mentioned above), it can offer a higher level of capacity.
On the contrary, it has been found that even if the capacity is made large by making large the total quantity of all acidic groups as above, it will involve a large extent of variation with time (capacity deterioration).